What are the symptoms of low quality guitar machine heads? I know it can be the nut that's causing the problem, but besides that, how can I know whether changing the machine heads will help my guitar stay in tune longer?

In my particular case, they seem to be O.K. but I am not sure. They feel just a little bit loose while tuning the guitar (compared to the other more expensive guitar I have), but I am unsure if something changes while playing or is it just the nut. Btw., the nut was changed once already some time ago because I wanted to put thicker strings and it did make a huge difference.

Suppose you tuned the guitar, left it alone for a day or two, and then picked it up again---is it still in tune? If not, that can't be the nut's fault, because you haven't played it. This isn't a definitive test, of course, but it might be suggestive.
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Alex BassonJan 25 '12 at 11:42

@AlexBasson it usually stays in tune after that. one or two strings maybe go just a tiny little bit out of tune, but I consider that to be normal.
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pootzkoJan 25 '12 at 12:05